1. Field of the Invention
The present invention relates to an improved process for the preparation of polyisocyanates with biuret structure which have excellent color quality and good monomer stability, by the reaction of aliphatic diisocyanates with certain carboxylic acids as biuretizing agent.
2. Description of the Prior Art
Aliphatic polyisocyanates which have a biuret structure, in particular those based on hexamethylene diisocyanate, have achieved worldwide technical importance for the production of lightfast and extremely weather resistant lacquers with high gloss retention. For use in this field, in particular for clear and white-pigmented coatings, the market demands colorless to only slightly colored products. Furthermore, for safe processing it is desirable that the portion of monomeric diisocyanates should be as low as possible and should not increase with prolonged storage. On the basis of toxicological investigation, processing is safe up to a maximum monomeric diisocyanate content of 0.7%, provided the usual protective measures for lacquer processing are observed. The above-mentioned limiting value has found its way into the literature (e.g., Memorandum "PUR-Anstrichstoffe" of the Haputverhandes der deutschen gewerblichen Berufsgenossenschaft and "Polyurethane Report" of the Paintmakers Association).
Numerous processes have become known in the course of time for the preparation of such polyisocyanates, but they all have special problems and disadvantages and do not, or only incompletely fulfill the above-mentioned requirements of the product. The following processes, for example, have been described:
Synthesis from diisocyanates and water, optionally in the presence of catalysts, see DE-PS No. 1,110,394, DE-OS No. 1,668,377, DE-OS No. 2,308,015, GB-PS No. 889,050, GB-PS No. 1,399,228, DDR-PS No. 140,744;
synthesis from diisocyanates and water in the presence of a solvent or a solvent mixture, see DE-OS No. 2,808,801, DE-OS No. 3,030,655;
synthesis from diisocyanates and water in which the water is brought to reaction in the form of steam, see DE-OS No. 2,918,739:
synthesis from diisocyanates and ammonia or ammonia-water mixtures, optionally in the presence of catalysts, see DE-AS No. 1,227,003:
synthesis from diisocyanates and amine, see DE-PS No. 1,165,580, DE-PS No. 1,174,759, DE-OS No. 1,568,017, DE-OS No. 1,963,190, DE-OS No. 2,010,887, DE-OS No. 2,261,065, DE-AS No. 2,438,258, US-P 3,824,266, DE-AS No. 2,609,995, DE-OS No. 2,803,103, DE-PS No. 883,504, GB-PS No. 1,263,609; see also Angew. Chem. 72, page 1002;
synthesis from diisocyanates and amine/alcohol mixtures, see DE-OS No. 2,654,745:
synthesis from diisocyanates and .omega.,.omega.'-diaminopoly ethers, see DE-OS No. 1,570,632, DE-AS No. 1,215,365;
synthesis from diisocyanates and substituted ureas, see DE-PS No. 1,101,394, DE-AS No. 1,227,004:
synthesis from diisocyanates and tertiary alcohols, optionally in the presence of catalysts, see DE-AS No. 1,543,178, DE-AS No. 1,931,055, DE-OS No. 2,308,015;
synthesis from diisocyanates and formic acid (DE-PS No. 1,174,760, DE-OS No. 2,308,015);
synthesis from diisocyanates and aldoximes (DE-OS No. 3,007,679).
Processes in which the diisocyanates are reacted with water are difficult to control on account of the inhomogeneity of the reaction mixture. Firstly, the processes are accompanied by the formation of extremely difficultly soluble polyureas which can only be dissolved by the application of high temperatures over a long period of time, whereby the color of the product is deleteriously affected. Even then, a proportion of these polyureas in some cases remains undissolved as a precipitate which is difficult to filter and must be removed by elaborate procedures before further processing is carried out. Furthermore, because of the volatility of most diisocyanates, deposits of urea are liable to form in the steam chamber of the reaction vessel. This also applies to processes in which water is used in vapor form.
When water is used as biuretizing agent, these deposits can only be avoided if solvents or solvent mixtures are used to homogenize the reaction mixture. These methods have, however, various disadvantages. Firstly, they require the use of large quantities of solvents which must subsequently be removed from the finished product by distillation, and secondly, very specific solvent mixtures of glycol ether acetates and phosphoric acid esters are required for colorless products. Furthermore, these processes require reaction temperatures of at least 140.degree. C. to avoid the intermediate precipitation of insoluble ureas. If such precipitates nevertheless occur, e.g. due to low reaction temperatures, it is necessary to employ temperatures of 160.degree. C. or more, as in the processes using water without solvents, if clear products are to be obtained. These high temperatures increase the occurrence of side reactions and lead to a marked loss in color quality.
Processes may be carried out without solvents if water is released in the course of the reaction from a compound which splits off water. These processes include in particular the technically important process using tert.-butanol as biuretizing agent. This process, however, also requires temperatures of about 180.degree. C., with all the attendant disadvantages for the quality of the product already mentioned above. Furthermore, the process entails the loss of biuretizing agent and release of combustible gases (isobutene).
The reaction of the diisocyanates with aldoximes is also characterized by the loss of the difficultly accessible biuretizing agent and the occurrence of highly volatile by-products (nitriles) which cannot be reused.
The reaction of diisocyanates with hydrogen sulphides gives rise to the toxic, low-boiling product, carbon oxysulphide, which also cannot be returned to the process and must be removed by elaborate procedures.
Common to all the processes mentioned so far is the fact that part of the diisocyanate is converted by reaction with the biuretizing agent into amines, i.e. the precursors of isocyanates. Processes have therefore been proposed for the direct conversion of diisocyanates to biuret polyisocyanates by reaction with amines.
However, due to the high reactivity of amines, these processes are accompanied by the formation of difficultly soluble polyureas even when highly developed mixing processes are employed, so that considerable application of heat is again necessary to dissolve these polyureas, and the high temperatures employed result in a deterioration in the color quality and increased formation of by-products. Not only uretdiones and isocyanurates but also carbodiimides and secondary products of carbodiimides are formed which have an adverse effect on the monomer stability of the end product.
The tendency to the formation of difficultly soluble polyureas may be reduced by the use of diamines whose reactivity has been significantly reduced by suitable means, e.g. by steric hindrance. The products, however, contain inter alia a high proportion of monomeric diisocyanates which have been formed from the diamines put into the process and cannot be removed by thin layer distillation.
Although the use of .omega.,.omega.'-diaminopolyethers gives rise to liquid polyisocyanates containing biuret, this solution to the problem is very expensive owing to the additional synthesis of biuretizing agent. Moreover, the ether groups present in these products result in poor weather resistance of the lacquer films obtained from them.
The formation of polyureas may be avoided by using monoamines or N,N'-disubstituted ureas, but the highly volatile monoisocyanates formed from these biuretizing agents must be removed from the reaction mixture. This can only be incompletely achieved, even at a high temperature, owing to the inevitable equilibrium reactions.
Although products with good color quality may be produced under mild conditions when diisocyanates are reacted with formic acid, these products still contain a high proportion of N-formyl groups. In order to obtain a polyisocyanate having a predominantly biuret structure, it is necessary to employ reaction temperatures of more than 160.degree. C. over a considerable time, thus causing a marked yellow discoloration. Moreover, the biuretizing agent is used up with the release of toxic carbon monoxide, causing considerable problems of effluent air disposal.
A process using diisocyanates and mixtures of amines and alcohols has also been proposed. Apart from other disadvantages, the products obtained from such processes are modified in their structure and have different properties. The same applies to products obtained from processes in which diisocyanates are reacted with ammonia.
It has now been found that polyisocyanates with biuret structure can be obtained with excellent color quality and good monomer stability by the reaction of aliphatic diisocyanates with certain monocarboxylic acids described in more detail below at comparatively low temperatures if the reactants are used in certain proportions.